1. Field of the Invention
The present invention relates to a method for manufacturing a glass blank for magnetic disk and a method for manufacturing a glass substrate for magnetic disk.
2. Background Information
Recently, a hard disk drive device (HDD) is incorporated in a personal computer or a DVD (Digital Versatile Disc) recording apparatus in order to record data. Particularly, in the hard disk device used in an apparatus such as the notebook personal computer based on portability, a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and magnetic recording information is recorded in or read from a magnetic layer using a magnetic head (DFH (Dynamic Flying Height) head) that is slightly floated on a surface of the magnetic disk surface. A glass substrate is suitably used as the substrate for magnetic disk because the glass substrate hardly plastically deformed as compared to a metallic substrate (aluminum substrate) and the like.
The magnetic head includes, for example, a magnetic resistance effect element, but such a magnetic head may cause a thermal asperity trouble as its specific trouble. The thermal asperity trouble is a trouble in which when a magnetic head passes over a micro-irregularly-shaped surface of a magnetic disk while floating and flying, a magnetic resistance effect element is heated by adiabatic compression or contact of air, causing a read error. Thus, for avoiding the thermal asperity trouble, the glass substrate for magnetic disk is prepared such that surface properties, such as the surface roughness and flatness, of the principal face of the glass substrate are at a satisfactory level.
As a conventional method for manufacturing a sheet glass (glass blank), a vertical direct press method is known. This press method is a method in which a lump of molten glass is fed onto a lower die, and the lump of molten glass (molten glass lump) is press-formed using an upper die. In the vertical direct press method, a period of time until the lump of molten glass (molten glass lump) is pressed after being supplied to the lower die is long, and therefore when the surface roughness of the lower die is improved, the molten glass lump is fused to the lower die (i.e. a state in which the lower die is in contact with the molten glass lump at a high temperature for a long time in a cumulative manner occurs, and the surface thereof is oxidized, so that the glass is seized). Accordingly, in the vertical direct press method, generally it is necessary that a mold release material such as, for example, BN (boron nitride) be attached to the lower die beforehand for preventing a situation in which the molten glass lump is fused to the lower die and cannot be removed. In this way, the mold release characteristic of the glass blank is secured. However, the surface of the glass blank obtained by press forming is projected because particles of the mold release material are attached thereto. Thus, even though the surface roughness of the forming surface of the die is improved, the surface roughness of the glass blank obtained is large owing to the attached particles of the mold release material.
After press forming, a shape processing process of cutting a glass blank along two concentric circles is performed for forming the glass blank obtained by press forming into a disk-shaped glass substrate. For shape processing, a method using a core drill or a method by scribing is employed. Generally, the method by scribing is better in terms of circularity and/or concentricity, but in the case where the surface roughness of the glass blank is large, the blade may be bounded, and the glass blank may be broken when cutting is performed because a cutting line may not be formed as a continuous circular line. That is, in the case where the surface roughness of the glass blank is large, the yield of scribing is reduced, and therefore the method by scribing cannot be employed in a mass production process. Therefore, in the case where the surface roughness of the glass blank is large, the method using a core drill is employed; however, a circularity and/or concentricity which is as high as that of scribing cannot be achieved due to fluctuations in circularity and rotation axis of the drill. Accordingly, the vertical direct press method, in which a glass blank having a large surface roughness is formed owing to attached particles of the mold release material, has a limitation in enhancing outer shape accuracy such as circularity while securing a high yield. In the vertical direct press method, pressing can be performed without attaching the mold release material to the upper die, and it is conceivable that a cutting line for scribing is formed in a surface formed in the upper die, but since particles of the mold release material are attached on a surface formed in the lower die, circularity and/or concentricity is deteriorated as well when the glass blank is completely cut.
From the viewpoint described above, Japanese Patent Laid-open Publication No. 2009-269762 discloses that a die in vertical direct press is formed so that a glass blank after pressing includes first and second cutting regions (regions to be cut by scribing) having a center line average roughness Ra of 0.1 to 50 nm in a concentric-circle shape, and other regions having a center line average roughness Ra of 100 nm or more.
Consequently, cutting lines for shape processing in first and second regions can be stably formed, so that a cut surface having defects such as chipping reduced can be formed, and the mold release characteristic of the glass blank during press forming is secured by means of regions having a center line average roughness Ra of 100 nm or more (relatively rough regions). Generally, for performing shape processing, a method using a core drill in place of a scribe cutter is also employed, but use of the scribe cutter allows shape processing of a circular shape having a higher circularity.